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As a bit of a primer for this guide, here is a short list of XML element types, to give some perspective as you work through the next sections. See: XML types, XML attributes and XML elements, later for a more complete list of WoW UI XML references. Keep in mind that there is no official Blizzard documentation for any of this. For description and properties of each element follow link below.

World of Warcraft has a fairly powerful layout engine for creating user interfaces. When combined with Lua for attaching behaviours to UI elements, this creates a flexible system with which the game's entire UI is created, as well as any custom AddOns. The XML and the WoW XML UI serve as a fundemental component of this. If you are unfamiliar with XML, consider visiting the XML Basics guide.

An XML file is a collection of elements (with a start and end tag), of which the User Interface files are no exception. There are two main types of elements that appear in the UI XML files. The first type are those that declare user interface items (or widgets), such as Buttons, Frames, Checkboxes. We will call these widget elements. The second type of element, which always occur inside the first type, define properties and behaviour of the widgets. We will call these property elements. Here is an example:

The Button element is of the first type, in other words a widget element. Its appearance in the XML file causes a Button with the name MyAddon_Button to be created. The elements inside it (such as Anchors, Anchor) define its properties, hence are of the second type. The general structure is always the same, you have elements representing widgets, and other elements inside them representing their properties. It can also happen that a widget element is inside another one. For example:

This example has two widget elements (Frame and Button), and several property elements (Anchors, Anchor). This creates a Frame, with a Button inside it. Here, MyAddon_Button is a child of MyAddon_Frame, and MyAddon_Frame is a parent of MyAddon_Button. Also note how the XML convention of abbreviating an empty element such as <Anchor point="CENTER"></Anchor> as <Anchor point="CENTER"/> is used.

Many of the elements (widget and property alike) can have attributes, such as the name attribute in the above examples.

A complete and valid XML file must contain exactly one element named UI, with some rather long attributes (best use copy/paste for this). Hence a minimal example of a complete UI XML file would be something like:

If you use the Blizzard Interface Customization tool to extract the contents of the Interface\FrameXML folder, you will come across the file UI.xsd which provides a schema definition for automated validation of your customized XML-files. Notice that XSD is an advanced XML feature and requires a special editor as well as a good knowledge of advanced XML-concepts. If you've already worked with XML you might find this very helpful to get started with WoW.

There is also an online validator available, which will take any XML you throw at it and check it against Blizzard's schema definitions. Unfortunately, it is not much use as a "getting started" reference, but rather it is much more useful to check what is going wrong should you lack access to the heavier tools that support XML editing with XSD support natively. Further, due to the way that the XSD was designed, it can some times throw errors which are not actually a problem. Be sure to check the FAQ for more information.

Every widget element may have the name attribute. If an element has a name, it causes a global Lua variable to be created with that name. This variable can then be used to call API methods on that widget. (See Widget API for a list of methods available for the different UI widgets.) Note that global variables are truly global across the entire UI, meaning that every name must be unique across all XML files.

Here is an example. Let's say in your XML file you have a section like this:

In any Lua code then you can use the variable MyAddon_Frame to refer to the frame and MyAddon_Button to refer to the button. For example, to show the frame, call MyAddon_Frame:Show(). Or to disable the button, call MyAddon_Button:Disable().

When defining the name of a widget, the special string $parent may be used. This will take on the name of whatever the parent of that widget is. For example:

Frames have a combination of a size and one or more anchors. For a frame to be laid out, the combination of these needs to define a rectangle on the screen in which the frame is to be laid out.

A size is specified using a Size element with either an AbsDimension or RelDimension child element.

Anchors allow for relative positioning, and also to allow frames to dynamically reposition their content based on resizing. A group of anchors is expressed via an Anchors element with one or more Anchor children, each of which may have an Offset.

This specifies a 100x100 frame anchored at the top left of the frame SomeOtherFrame, and offset by 10 pixels to the right and 10 pixels down. (Note that the Y axis increases from the bottom up, so negative Y coordinates indicate downwards movement).

Note that no Size is specified here; the size and location of this frame is defined entirely by its relationship to SomeOtherFrame. In particular, it will be inset by 5 pixels from the top left and bottom right of SomeOtherFrame. As SomeOtherFrame changes size, our frame will change size as well.

There are five levels of layers, although in practice you will often only assign objects to three of the five. Of these three, BACKGROUND is in the back, ARTWORK is in the middle and OVERLAY is in front. The only way to be certain of where and how much of your object is visible is to assign it to a specific layer.

Templates are used when you want to create a common layout for several frames. In this way you can save on the amount of code needed to recreate each frame, as the frames will automatically take on the properties, children, and attributes from whatever template it inherits. There are several rules that must be following when initially creating the template:

Templates are created at the root of the file. Meaning that you cannot have a template that is a child of another element

Templates must have their virtual attributes set to true

Templates must have a name, and it cannot use the $parent keyword

Children of a template do not need to be named, but when doing so, you should use the $parent keyword

As discussed above, there is a special keyword that can be used when naming something inside the template. When you inherit a frame, any object inside the template that is inherited, which has the keyword $parent, will automatically replace the keyword with the name of the parent which inherited the template. Example:

When the XML file is read, the contents of the mycode.lua file is read and interpreted. The code in that file is then available for the rest of this XML file, and any XML files that are read after this one. Functions that are defined in mycode.lua can be called from event handlers in the XML file.

If you are not used to event handled programming, read this section, otherwise you can skip right through it.

Traditional programming style involves writing a program that has a start and finish. Execution starts at the beginning, and after following through the algorithm in the program, it eventually gets to the end.

Event handled programming is somewhat different. In this case, you create several, independent sections of code, with no clear start or finish. Each of these sections of code are designed to be executed in response to a certain event occurring. Such a section of code is called an event handler. An event handler may get executed once, many times, or maybe even never. This kind of programming style is used whenever you want your code to interact with an external program, and do things in response to whatever that program is doing.

In the case of World of Warcraft, the core game is external to the interface code. Whenever something happens in the game, it asks the interface code to respond to it, by calling one of the event handlers.

This frame has one event handler only, which is executed every time this Frame becomes visible. The contents of the OnShow element can be any valid Lua code. In this example, it's a single statement, which calls the function message, which causes a dialog box to pop up with "Hello" inside it. (message is a function defined in BasicControls.xml and is available by default.)

Here is a more complete example of how you include Lua code and then reference it from the event handlers.

When MyModMainFrame gets loaded (which happens once, at the start of the game, when all the XML files are read in), the OnLoad handler gets executed. In this case, the single Lua statement there simply calls the MyMod_ShowMessage function.

It is recommended that you prefix the names of all of your functions, UI objects and global variables with the name of your AddOn like above; all data is shared between Blizzard's own UI and all your addons, so picking a unique prefix helps avoid clashes with other people's code.

For the above example to actually work, you must either create an AddOn with the mymod.lua and mymod.xml files, or include them in the FrameXML directory and reference myframe.xml from FrameXML.toc. See AddOns on how to combine these files into an AddOn.

Widget Handlers is a complete reference of what event handlers can be used by each object type.

Note: the OnEvent handler is special (see below).

You will notice that the event handlers in the list mostly cover UI related events. The OnEvent handler is special in that it is a general, collective handler for a large number of other events. It gets called for any of the remaining hundreds of game related events. See Events for a list of game events and how to register for them and handle them.

The default user interface that is visible without any installed third-party addons is built into the MPQ files. It can't be modified because it is protected by a digital signature, but using the World of Warcraft Interface AddOn Kit (US, EU) provided by Blizzard, the files can be extracted to see how the interface works.